During the production of metal strips by rolling, it is necessary to pickle the strips, particularly in order to remove any oxides that may form on their two faces. Generally, the strip is run continuously through an acid solution bath contained in several long, oblong tanks. The horizontally-fed strip passes over a threshold at the upstream end of each tank and enters into the tank forming a loop which dips into the treatment bath. The strip leaves the tank at the other end by passing over a threshold and then dips into the following tank.
In view of the high feeding speed of the strip, a substantial length of it must be immersed, and this is why the strip is made to dip successively into several tanks placed in sequence. The assembly is linked to an acid supply and regeneration circuit which makes it possible to vary the strength of the acid in the tanks from upstream to downstream according to the degree of pickling of the strip.
Such installations are very bulky and difficult and time-consuming to maintain, especially because of the risk of acid corrosion, which calls for regular replacement of certain parts. The bath also gives off acid vapors which can corrode the framework of the building and devices installed in the vicinity of the tanks. While each tank is of course covered by a cover and vapors are collected, the risk of corrosion cannot totally be avoided.
Moreover, a huge amount of acid is consumed because of the large quantities involved and the need to regenerate the acid in order to keep it at the prescribed strength in each tank.
To overcome these drawbacks, JP-A-61.235582 has already proposed replacing the succession of tanks normally used by a closed vessel fitted with deflecting rollers around which the strip passes following a zigzag path, and comprising several superposed "outward" and "return" sections passing between ramps for injecting liquid. While such an arrangement makes it possible to reduce the spatial requirements of the installation, a substantial amount of acid vapor is given off. Moreover, the strip is not immersed in the liquid, which is simply sprayed over each section and drained by gravity.
U.S. Pat. No. 3,473,962 describes another type of zigzag installation in the form of a vessel containing several superposed tanks filled with liquid and associated with deflecting rollers placed at their ends. The strip first passes into a lower tank and travels, at the opposite end, around a deflecting roller whose lower portion dips into the liquid and whose upper portion is placed at the level of the input into the next tank placed above. The strip therefore moves upwards from one tank to the next up to the vessel's output.
The bottom of each tank is inclined in such a way as to ensure that the treatment liquid flows in the direction opposite to the product feed direction. The liquid enters the upper tank at the strip output point and leaves the tank at the opposite end by falling into the tank below it and so on.
Moreover, the fall of the liquid is broken by vertical dividing walls that dip into the liquid, with the resulting turbulence improving the efficiency of the treatment. However, the liquid can only be renewed slowly since it flows by gravity from one tank to the next, with turbulence being relatively limited.
The object of the invention is also a zigzag installation in which the strip passes into superposed tanks, but one using much simpler arrangements, which considerably reduce the risks of corrosion while considerably improving the efficiency of the treatment.